1. Field of the Invention
The present invention relates to a terminal unit and a method of connecting terminal units in an ATM switching system.
2. Description of the Related Art
Recently, high data rate communication networks such as a B-ISDN (Broadband Integrated Services Digital Network) for transmitting various types of communication data have been put to practical use. The network transmits data at an extremely high data rate of about 1.2 G bit/s (gigabits/second). An ATM (Asynchronous Transfer Mode) system has already been developed for putting faculty of much high data rate transmission to practical use. The MSSR (Multi-Stage-Self-Routing) switching method used in the ATM system comprises a switch (inter-network connection relay line switch) for switching lines from a high data rate communication network and terminal units (trunks), connected to the high data rate communication network (relay line), for terminating the high data rate communication network.
Generally, a plurality of terminal units are serially connected to one switch as shown in FIG. 1. An ATM cell (a 32-128 byte fixed-length signal block comprising a header and an information field containing quantized data) is transmitted at data rate of 1.2 G bit/s through the high data rate lines connecting terminal units 81, 82 and 83 to each other, and connecting terminal unit 83 to a switch unit.
Low data rate lines capable of processing data at, for example, 1.5 M (mega), 2 M, and 150 Mbit/s, etc. are connected to each of the terminal units 81, 82, or 83. Each terminal unit receives necessary cells through the high data rate lines, and distributes and transmits the cells to the low data rate lines at a data rate depending on the capabilities of the low data rate lines connected to the terminal unit. Each terminal unit also multiplexes cells transmitted through the low data rate lines and transmits them to a broadband communication network, etc. through the high data rate lines through the switch.
The functions of the terminal units can be performed using a token cycling through a plurality of serially connected terminals and a buffer temporarily storing a plurality of cells provided in a multiplexing circuit of each terminal unit. That is, when data are transmitted from the low data rate lines to the high data rate lines, terminal unit 81, 82, or 83 has the right to transmit its own cells if it has obtained the token. For example, if the terminal unit 81 has obtained the token, terminal unit 11 outputs its own cells to a 1.2G high data rate line (terminal unit connecting cable) 95, and releases and sends the obtained token. Therefore, the succeeding terminal unit 82 positioned downstream receives the above described cell as well as the token. Terminal unit 81 sends only the token to the succeeding terminal unit 82 if it has no cells in its own buffer.
Likewise, upon receiving a token, the terminal unit 82 sends a cell (if any in its own buffer) to a high data rate line (terminal unit connection cable) 96 after adding its own cells and token to the end of the cells of the preceding terminal unit 81. If there are no cells in its own buffer, the terminal unit 82 sends the cells of the preceding terminal unit 81 with only a token added to the cells.
Terminal unit 83 sends a cell (if any in its own buffer) to a high data rate line (switch connection cable) 97 after adding the cells to the end of the cells of the terminal units 81 and 82. However, terminal unit 83 sends a token to terminal unit 81 through a high data rate line (terminal unit connection table) 94.
The above described process is repeated, thus transmitting multiplexed cells to the switch. If a token has been cleared by an error, etc., then only terminal unit 81 is responsible for generating a new token.
The configuration of the above described terminal units are further explained as follows. As shown in FIG. 2, each of the terminal units comprises four connectors a, b, c, and d. The terminal units 81, 82, and 83 are serially connected to switches with high data rate lines (switch connection cables) 91 and 97 and high data rate lines (terminal unit connection cables) 92-96 connected to these connectors a, b, c, and d.
When cells are output to a high data rate line, the cells are transmitted through the path of a switch, the high data rate line (switch connection cable) 91, connectors a and b of terminal unit 81, the high data rate line (terminal unit connection cable) 92, connectors a and b of terminal unit 82, the high data rate line (terminal unit connection cable) 93, and connector a of terminal unit 83. Then, necessary cells are received by the terminal unit 81, 82, or 83.
On the other hand, when cells are output to a low data rate line, cells output by each of the terminal units are transmitted through the path of connector d of terminal unit 81, the high data rate line (terminal unit connection cable) 95, connectors c and d of terminal unit 82, the high data rate line (terminal unit connection cable) 96, and connector c of terminal unit 83 together with a circulating token or a token generated by terminal unit 81. Multiplexed cells are separated from the token by terminal unit 83 and transmitted to a switch through the high data rate line (switch connection cable) 97. Then, the token is returned through the path of connector b of terminal unit 83, the high data rate line (terminal unit connection cable) 94, and connector c of terminal unit 81. The token then starts circulating again, as shown in FIG. 1 after exiting from connector d.
Thus, if a token is lost due to an error, etc. and terminal unit 81 recognizes that it is the highest order terminal unit, a new token should be generated by terminal unit 81. Then, terminal 82 unit should recognize that it is an intermediate terminal unit, and send downstream a cell with a token. Likewise, terminal unit 83 should recognize that it is the lowest order terminal unit, separate a token from a cell, and send it back to the highest order terminal unit.
Since the operation of each terminal unit depends on its position in a connection sequence, each terminal unit is assigned a different operation. The operations are set by a plurality of dip switches such that each of them is assigned to a corresponding position in the connection sequence.
However, with the above described conventional connection, a troublesome reorganization must be performed if a new terminal unit is added to terminal unit 83 shown in FIG. 1 as follows. First, the existing terminal unit connection cable 94 between the highest order terminal unit 81 and the lowest order terminal unit 83 is removed and set to connect the highest order terminal unit 81 to the new terminal unit. Then, the switch connection cable 97 is removed from terminal unit 83 and connected to the new terminal unit. Furthermore, terminal unit 83 is released from the operation of the lowest order position and newly assigned an operation for an intermediate position. Thus, adding a terminal unit requires more than a few process steps, and causes a very difficult problem with connection changing.
If a replacement or additional terminal unit is assigned a wrong operation in the connection sequence, it erroneously functions, and does not perform a correct operation, according to the wrong assignment after the replacement or addition. Consequently, there is the problem that the system may malfunction after the replacement or the addition.